Abstract

High-numerical-aperture zone plates have important applications in high-resolution optical maskless lithography as well as scanning confocal microscopy. We describe two methods to experimentally characterize the focusing properties, i.e., the point-spread function, of such diffractive lenses. The first method uses spot exposures in photoresist and the second uses a conventional knife-edge scan. The experimental results agree well with rigorous theoretical calculations.

Figures (10)

(Color online) Schematic of ZPAL. An array of high-numerical-aperture diffractive lenses (for example, zone plates) is used to create an array of tightly focused spots on the photoresist-coated substrate. By scanning the substrate while modulating the intensity of each spot, patterns of complex geometries can be printed in a dot matrix fashion. Inset: Images of an array of phase zone plates of NA=0.7. These zone plates were fabricated in fused silica and were designed to focus λ=400nm at a focal length of 40μm (the printing of the digitized image creates moiré artifacts).

(Color online) (a) Schematic illustrating the nonlinear response of the photoresist. Single-spot exposure at a given dose samples the PSF for an ideal resist response of thresholding. (b) Exposure doses. The samples were chosen such that they are closely spaced near the center of the spot and are more widely spaced away from the center. This provides high resolution near the center and minimizes the total number of samples.

(Color online) Same data as in Fig. 3, but the intensity is plotted in log scale to emphasize the correlation between the theoretical and experimental values even for points far from the center of the spot. The simulation data are plotted by the dashed curves.

(Color online) Measured image contrast as a function of grating period. Single-pixel lines refer to lines formed by a single pass of the focused spot, while double-pixel lines refer to the case in which each line is formed by two overlapping passes.

(Color online) Knife-edge scans for zone plates of three NAs. Detector signal is plotted as a function of scan coordinates in X and Y for zone plates with NAs of 0.7, 0.8, and 0.85. The transition from bright to dark in the signal is sharper for higher NA as expected.